1. Field of the Invention
The present invention relates to a semiconductor device wherein an insulating substrate carrying a semiconductor element is mounted on a heat sink, and specifically to a semiconductor device of a low price and a long life.
2. Background Art
FIG. 5 is a sectional view showing a conventional semiconductor device; and FIG. 6 is an enlarged sectional view of the chief part of the semiconductor device. An insulating substrate 11 has an upper conductor 12 formed on the upper surface thereof and a lower conductor 13 formed on the lower surface thereof. A semiconductor element 14 is mounted on the insulating substrate 11 with an under-element solder 15 therebetween. The insulating substrate 11 is mounted on a heat sink 16 with an under-substrate solder 17 therebetween. On the periphery of the heat sink 16, a case 18 is fixed by an adhesive 19, and inside the heat sink 16, a silicone gel 20 is injected for securing insulation.
There has been proposed a technique to improve the heat dissipation of a semiconductor chip by filling an insulating liquid having high thermal conductivity in a heat-radiating case accommodating a semiconductor chip and a heat sink (For example, refer to Japanese Patent Laid-Open No. 63-096945).
By the repetitive heating and cooling due to the operation of the semiconductor element 14, the insulating substrate 11 and the heat sink 16 are expanded and contracted. Thereby, when the semiconductor device is used for a long period of time, cracks 21 are produced in the under-substrate solder 17. Since the portions of the cracks 21 constitute air layers that have very small thermal conductivity, heat transfer from the semiconductor element 14 to the heat sink 16 is lowered. There was a problem wherein the semiconductor element 14 was abnormally heated, the semiconductor device was destroyed, and the life of the semiconductor device was shortened. In the technique of Patent Document 1, an insulating liquid had to be used, and a conductive substance having high heat radiation could not be used. Therefore, heat radiation was insufficient.
In contrast, when a long life was required, a low linear expansion material, such as aluminum silicon carbide (AlSiC) and copper molybdenum (CuMo), was used in place of copper (Cu) generally used as the material for the heat sink. In this case, although cracks were not easily produced in the under-substrate solder, there was a problem of high material costs.